(a). Field of the Invention
The present invention relates in general to a communication system, and more particularly to an apparatus for estimating and compensating sampling timing offset in a multi-carrier system and a method thereof.
(b). Description of the Prior Arts
In recent years, multi-carrier technology is widely applied to high-speed communication systems, such as asymmetric digital subscriber loop (ADSL), IEEE 802.11 a/g wireless local area network (WLAN), etc. FIG. 1 is a block diagram of a typical multi-carrier system 100. The transmitter of the multi-carrier system 100 first distributes the data under transmission into N frequency-domain subchannels (N=2n, n is an integer) via a signal mapping unit 101, and maintains the orthogonality among the signals of each subchannel to prevent inter-carrier interference (ICI). Next, an inverse fast Fourier transform (IFFT) device 102 is used to transform the subchannel signals into time-domain signals, to which a guard interval (GI) is added by a GI adding device 103. Then, each of these time-domain signals is passed through a parallel-to-serial converter (P/S) 104 and a digital-to-analog converter (DAC) 105, and then transmitted via a channel 106. The receiver of the multi-carrier system 100 first uses an analog-to-digital converter (ADC) 107 to sample the received time-domain signals. Next, the guard interval of the sampled signals is removed by a GI removing unit 109. The result thereof is provided to a serial-to parallel converter (S/P) 110, and then a fast Fourier transform (FFT) device 110 is used for transforming to frequency-domain signals. Last, the receiver compensates these frequency-domain signals by a channel compensator 112 and performs signal demodulation via a signal demapping unit 113 to recover to the original transmitted data.
A set of N-point IFFT output is typically called a symbol. Since the channel impulse response (CIR) is usually not ideal, a received symbol after passing through the channel 106 would impact the reception of subsequent symbols, i.e. inter-symbol interference (ISI). To prevent ISI, an additional guard interval (GI) is added between two symbols. Two typical ways to implement the guard interval are zero-padding (ZP) and cyclic prefix (CP). In ZP, a string of zero is added as the guard interval and energy efficiency is thus improved. In CP, a latter portion of a symbol is copied and put before the symbol as the guard interval. CP can reduce the ICI resulted from the channel impulse response. The circuits 103 and 109 of FIG. 1 are used to add and remove the guard interval respectively.
When demodulating the received time-domain signals, the receiver of the system 100 needs to transform them into frequency-domain signals by the FFT device 111 and performs the demodulation within each subchannel respectively. If synchronization error exists in the time-domain signals inputted to the FFT device 111, then additional ICI and phase rotation would be generated in the outputted frequency-domain signals to damage the orthogonality of the outputted frequency-domain signals. For a multi-carrier system, the synchronization error mainly results from sampling frequency offset and sampling phase error. Besides the additional ICI and phase rotation, the sampling frequency offset would generate an accumulated sampling timing offset, which may cause ISI to degrade the system performance.
In view of this, the present invention provides an apparatus and a method that can estimate and compensate the accumulated sampling timing offset by using pilot signals of a symbol, thereby upgrading the performance of a multi-carrier system.